Emission and Transmission of Heat 39 



A' is a coefficient depending on the form and the dimensions of 

 the body, / is the excess of temperature in degrees Centigrade. 



791. When the excess of temperature is but slight, one 

 may neglect the terms of the second degree and we then have for 

 the total quantity of heat emitted 



M=R + A = (K+l?}t = Qt 

 which is Newton's L,aw. 



The formulas (#) and () have only been proven for ex- 

 cesses of temperature ranging between 45 F. and 117 F. for 

 greater excesses of temperature we must employ Dulong and 

 Petit 's formulas. 



We will therefore enunciate the formulas in a general form 

 and give the values of the coefficients K and K' for different sur- 

 faces and bodies of different form according to the results of our 

 experiments. 



General Formulas Relative to the Emission of Heat in Air. 



792. The quantity of heat emitted by a surface maintained 

 at constant temperature depends on the radiation and the contract 

 of air. If we designate by Mthe total quantity of heat emitted 

 in a certain time, by R and A the quantities respectively due to 

 radiation and air contract we have ; 



M=R + A (i) 



793. Heat Emitted by Radiation. The quantity of heat 

 emitted by radiation, for unit surface and per unit of time, is in- 

 dependent of the form and size of the body provided that its 

 surface has no reentrant portions ; it depends solely on the nature 

 of the surface, on the excess of its temperature over that of the 

 objects to which radiation takes place, and on the absolute value 

 of the temperature of these objects. 



794. When a body is surrounded by objects having dul 

 surfaces, which is nearly always the case except in laboratory 

 experiments, the quantity of heat R is given by the formula : 



^=124.72 A^O' i) (2)* 



Where represents the temperature of the surrounding objects 

 / the excess of the temperature of the surface above that of the 

 surroundings, a a constant having the numerical value 1.0077, 



* All the quantities in formula (2) and (3) are in French units. 



